ABSTRACT
This chapter deals with fatigue cracks in bodies whose material is linear elastic. Under arbitrary loading and/or in the case of arbitrary geometry, fatigue cracks propagate in mixed modes. Actually, to evaluate the damage fields along crack trajectories, one needs to know stress-strain fields in cracked bodies. The simplest problem is when a mixed-mode crack propagates in its initial plane, i.e., without kinking or branching. It is, for example, the case of cracking between two bodies whose plane surfaces are glued or welded with a layer that is weaker than the materials of both bodies. The equivalent or very close criteria postulate that the angle of the first crack path corresponds to zero circumferential tangential stresses, to maximal stress intensity factor in mode I, or to zero stress intensity factor in mode II for a kinked crack tip.
